1. Field of the Invention
This invention is related in general to the field of computer systems. In particular, the invention consists of a programmable logic device that implements a state machine to perform analog-to-digital conversion of a capacitively terminated signal.
2. Description of the Prior Art
Using traditional digital logic standards such as transistor-transistor-logic (xe2x80x9cTTLxe2x80x9d), it is only possible to detect two conditions, i.e., a logic-high or a logic-low state. While modern digital devices have the ability to detect a third state, i.e., a high-impedance state, digital detection-devices typically utilize each detection line (xe2x80x9csense linexe2x80x9d) to indicate only the traditional logic-high and logic-low states. This poses severe limitations when it is desired to detect three or more conditions. For example, a typical system that includes a connector designed to accept plug-in cards can only identify two different types of cards utilizing a single sense line. Additional types of cards can only be differentiated by utilizing more than one sense line. Using a binary detection system, the number of conditions that can be detected Nd is equal to the number two raised to a power that is equivalent to the number of sense lines Ns:
Nd=2Ns.
In this manner, one sense line can detect two conditions, two sense lines can detect four conditions, and three sense lines can detect up to eight conditions. Returning to the example of a system with a connector for accepting plug-in cards, the ability to detect twenty different types of cards would require the use of at least five sense lines (four sense lines is insufficient as two raised to the power of four can only identify sixteen disparate conditions). Using multiple sense lines is problematic as it increases the number of signal traces on a system""s circuit board and the number of connector pins dedicated to plug-in card identification. This requires a larger circuit board and connector or a reduction in the available resources for other signals. Additionally, plug-in cards accomodating multiple sense lines also require an increased number of electrical traces and connector pins.
In U.S. Pat. No. 4,939,518, Massao Hotta et. al. disclose utilizing an analog to digital converter (xe2x80x9cA/D converterxe2x80x9d) to sample an analog voltage signal at N different voltage levels. Using a cyclic averaging A/D converter, the analog voltage signal is sampled and input into numerous comparators. If applied to the connector/plug-in card example, the Hotta device would require only a single detection line and associated connector pin.
An important aspect of Hotta""s invention is a cyclic averaging A/D converter that can be readily implemented with integrated circuits (xe2x80x9cICsxe2x80x9d). A voltage-dividing circuit generates reference voltages at N levels. The generated reference voltages are divided into groups of M elements and N+N/M switches are used to input the reference voltages into N/M comparators.
This approach requires an excessively large number of switches and comparators. Additionally, each reference voltage level is associated with a resistor in the voltage-divider circuit. While the switches and comparators may be implemented with either analog or digital devices, resistors are extremely problematic to implement in the digital domain. Accordingly, the Hotta A/D converter requires the use of a relatively large amount of circuit-board space for the implementation of switches, comparators, and the voltage-dividing circuit. Accordingly, it would be advantageous to utilize an A/D converter to detect numerous conditions on a single detection line without the need for a disproportionately large number of comparators, switches, and resistors.
The invention disclosed herein is a system implementing a state machine to detect a large number of disparate conditions on a single sense line. The state machine is an implementation of an A/D converter that measures the time that is required for the sense line to go from a ground/logic-low state to a threshold voltage/logic-high state. The state machine and associated sense line may be utilized to identify one of numerous disparate plug-in cards inserted into a connector. Each plug-in card includes a termination circuit that includes either a grounding device or a terminating capacitor. Multiple sense lines may be utilized to simultaneously identify multiple plug-in cards inserted into a multitude of connectors.
One aspect of this invention is that each type of plug-in card may include a terminating capacitor of a unique value. A connector may be designed to accept multiple different types of plug-in cards such as modems, network interface cards, video graphic adapters, and audio input/output cards. Each type of card may utilize a terminating capacitor of a certain value or within a specified range of values. For example, modems may be associated with a capacitor value of approximately ten micro-farads while video graphic adapters may correspond to a capacitive value of about twenty micro-farads. Additionally, models of plug-in cards may be identified within type groups. Modem plug-in cards may be assigned a range of values, say five to fifteen micro-farads. A particular model of modem may be identified by using an assigned capacitive value within that range.
An advantage of this invention is that it can be implemented using programmable logic devices such as field-programmable gate arrays (xe2x80x9cFPGAsxe2x80x9d) or complex programmable logic devices (xe2x80x9cCPLDsxe2x80x9d). This locates the A/D conversion process within a digital IC, eliminating the need for a large number of switches, comparators, and resistors.
Another aspect of this invention is the ability to utilize additional state machines to implement a fully redundant detection system. While a single state-machine may be connected to numerous sense lines, only one A/D conversion may take place within that state machine at any given time. If multiple plug-in cards are inserted at approximately the same time, only the first plug-in card may be initially identified by a single state-machine. Identification of subsequent plug-in cards must wait until the state machine becomes available. However, additional state machines may be used to simultaneously identify other plug-in cards. Because multiple A/D conversions should not occur on the same capacitor, a redundant A/D conversion begins with the state machine checking a handshake signal to determine if another state machine is performing a conversion on the same sense line.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention comprises the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose just a few of the various ways in which the invention may be practiced.